Determination of fuel volatility may be useful for various aspects of engine control and diagnostics. While sensors may be used, the inventor herein has recognized an advantageous approach for estimating fuel volatility in place of, or in addition to, other detection techniques.
In one example, it is possible to determine the fuel volatility and/or combustion characteristics based a rate of change of vapor generation during fuel vapor purging operation. In one example embodiment, the rate of change of vapor generation, (which may be normalized based on mass air flow, manifold pressure, or other parameters to reduce the effect of variable flown a sensor) can be used to enable diagnostic fuel tank operation after the engine is stopped. In this way, it can be possible to reduce errors in degradation detection. For example, in the case where a sealed fuel tank is checked for leaks based on environmental temperature changes, changes in pressure due to high fuel volatility that may mask such leaks can be identified. Furthermore, a improved measurement or estimate of fuel volatility can be used for other engine control purposes as well.